An apparatus for determining a physical process quantity of a medium is usually also called a sensor. Such sensors are generally used in industrial processes for determining information about that process. Depending on the qualities of the physical process quantity, or the information to be obtained, various measuring principles are relied upon.
In the context of the present invention, the term “physical process quantity” is to be understood to mean, for example, the filling level of a medium in a vessel, the pressure of a medium in a vessel, the flow of a medium through a conduit, the temperature, the density of the moisture, or any other material constant of a medium. However, the present invention is not limited to the above explicitly mentioned process quantities, but is also applicable for devices for determining other physical process quantities.
The information obtained by means of various measuring methods essentially serves to monitor and to control (with or without feedback) a process. The filling level of a filling matter may thus be measured as follows: capacitive filling level measurement, filling level measurement based on a pressure measurement, filling level measurement using ultrasonic waves, filling level measurement using radar, filling level measurement using a guided microwave, filling level measurement by means of vibration and conductive filling level measurement (limit level measurement).
In capacitive filling level measurement the filling matter and the vessel are combined with a measuring probe to form an electrical capacitor. The filling level is detected by measuring the capacitor's capacitance.
In non-contact ultrasonic filling level measuring methods, ultrasonic pulses are generated. A piezo-ceramic ultrasonic transducer transmits periodic sound pulses that are reflected by the filling matter surface. Using the combined transmitting and receiving system, the filling level is computed from the measured delay of the sound.
When using a guided microwave, radio frequency microwave pulses or electrical pulses are guided along an electrical waveguide, such as a steel cable or rod. On impact with the filling matter surface, the impulses are reflected. The delay of the pulses is evaluated by the integrated electronics and output as the filling level. This method is often also known as the TDR method.
In radar based filling level measurement, the delay is measured between transmitting and receiving very short microwave pulses. Time is a measure of the filling level, while the filling matter surface directly acts as a reflector.
In the filling level measurement by means of vibration, a vibration sensor is made to vibrate using piezoelectricity. On contact with the filling matter, the vibration is attenuated. The measuring electronics detects when the limit level is reached.
Finally, in the conductive filling level measurement, when a filling matter contacts the measuring probe, a current circuit is closed and a switching command is initiated. The conductive measuring principle is for economically detecting limit levels in electrically conductive fluids.
Pressure based measuring techniques include, for example, the method using process pressure detection or using differential pressure detection. In the method using process pressure, the pressure in tube conduits or vessels is detected using an oilless metallic or ceramic measuring cell, and converted, for example, into a 4-20 mA current signal. According to the differential pressure principle, the differential pressure is measured preferably using ceramic or metal measuring cells, and converted preferably into a 4-20 mA current signal.
With filling level sensors in the sense of the present invention, a basic distinction may be made between continuously measuring sensors and limit level sensors. Limit level sensors do not determine the degree to which a vessel is full, but detect when a predefined filling state is reached.
Continuously measuring sensors use for example ultrasonic or microwave based, capacitive and pressure based measuring techniques. Limit level sensors use methods such a vibration and capacitive measuring techniques.
For fulfilling specific tasks and in view of the application conditions, a sensor must fulfill certain requirements and/or fulfill certain industrial standards. These standards comprise requirements with respect to the resistance of the sensors to rough industrial environment conditions and/or to the media to be measured. A further requirement is the ability to be attached and the adaptation of the sensors to the process. Moreover, requirements with respect to the electrical connection of the sensors, the output of the information obtained, and the adherence to certain safety rules of the sensors such as explosion protection rules (“Ex zone separation”) must be met.
In order to adhere to these and other standards, a certain basic functional structure of each sensor is obtained that may be characterized by certain functional units. These are, among others, a sensor unit, an evaluation unit, an output unit, a voltage supply unit, an attachment unit, also called process connection, and a housing unit.
The sensor unit based on a mechanical or electromechanical principle, i.e. the sensor element contained therein, converts the physical measuring quantity into an electrical quantity while directly or indirectly contacting each medium. The electrical measuring signal thus generated, which is representative for each physical measuring quantity, is then further processed by the sensor electronics unit. The sensor electronics unit is a sensor specific circuit unit and must therefore be adapted to each sensor element. The sensor electronics unit processes the electrical measuring quantity signal in such a way that, for example, the electrical measuring signal is amplified, filtered and converted into a digital measuring signal.
The sensor electronics unit has a series connected evaluation unit deriving, with respect to the medium, the desired information from the measuring signal processed by the sensor electronics unit. The measured value thus generated is forwarded by the sensor via an output unit series connected after the evaluation unit to a process control system, for example, via a field bus or a two-wire loop. The sensor unit, the sensor electronics unit, the evaluation unit, and the output unit are often accommodated in a single housing and are supplied with voltage or current by a voltage supply unit also accommodated in the housing. The housing and/or the entire apparatus is attachable to a vessel or wall portion via an attachment unit, also called a process connection.
As already mentioned, each sensor must comply with a certain safety policy encompassing all these functional units. Therefore, a safety policy must be established for each sensor, taking into account different sensor components or their design.
Due to the multitude of different measuring quantities and to the measuring techniques for converting physical quantities into electrical information, it has been the usual practice to develop and manufacture for each measuring task a specific sensor with a unique design and functional units as well as mechanical components specially adapted to each measuring task. By this individual and sensor specific device development and manufacture, sensors may be provided that are optimally adapted to their measuring tasks and the application conditions.
A great disadvantage of such an approach is the high development cost and the great number of components and functional units that must be developed in connection with each sensor generation. This high development complexity, however, means high costs for manufacturers who do not specialize in a single sensor but offer a whole range of possibly related, but otherwise, different sensors. Also on the side of the users, the multitude of components and functional units may contribute to an increase in the cost required if, for example, the operation or the attachment of each sensor is different.
In our view, an apparatus for determining a physical process quantity should be known from U.S. Pat. No. 6,295,874 B1 and the associated WO 01/18502 A1, in which the process quantity is determined using a delay method. The apparatus shown herein may comprise an evaluation unit which is to be essentially independent of the sensor used. In our view, however, said document teaches the use of a sensor operating on the delay principle. Moreover, a communication unit independent of each sensor is used for data exchange with a remote process control. It must be recognized, however, that the communication unit is intended to be combined with a sensor based on the principle of delay measurement.
A prospectus of the German firm Krohne/Deutschland does show a modular structure of a filling level radar sensor, wherein various sensor units of a filling level radar sensor, such as horn antenna, wave guide or wave stick are combined, using a spacer, with two different sensor housings containing the respective electronics. Again, according to the technical understanding of the inventors, only a certain variability in the manufacture of a filling level radar sensor is taught.
Finally, the integration of absolutely oilless measuring cells in a modular system of process connections, housings, signal transmission methods, matching approaches, combined with the convenience of use provided by an intelligent system of operation are known from an offprint entitled “Lego für Erwachsene” (“Lego for grownups”) by Rolf Hauser, in the journal Messtechnik, Steuem, Regeln, Automatisieren, Messen, August 1999. In it various pressure sensors are presented that can be combined with different housings. Again, modularity seems to be limited to one measuring principle only.
Finally, in order to provide a better explanation and understanding of the present invention, a definition of terms used in the present specification is given. The present definitions will provide at least a rough idea for interpreting the units and components of a sensor described. For interpreting the terms, however, the technical knowledge and the insight of a person skilled in the art in the field on which the present invention is based may also be necessary.
Sensor:
An entire apparatus for determining a physical process quantity of a medium. A sensor may be, for example, an apparatus ready for attachment or operational for determining a physical process quantity of a medium.
Sensor Element:
An element directly or indirectly contacting the medium to be measured in order to sense the desired physical process quantity. This may be an electromechanical element comprising, for example, one or more assemblies or components for converting a physical process quantity into an electrical signal. The conversion may also conceivably be carried out under the influence of an electrical excitation. Sensor elements include various antennas (horn antenna, rod antenna, patch antenna and the like), cable or rod probes for TDR, tuning forks with piezo drive, pressure measuring cells, differential pressure measuring cells, cable or rod probes for capacitive measurement and ultrasonic transducers.
Sensor Electronics Unit:
An electronic assembly, such as a circuit with electronic components, possibly including associated software for processing the electric signal from the sensor element and deriving a measured value that characterizes the process quantity.
Examples for a sensor electronics unit and its components for each measuring technique are listed below:
a. Radar: transmitting pulse generator, sensing pulse generator, frequency control for clock oscillators, sampling receiver, amplifier, filter, logarithmic amplifier, A/D converter, ECHOFOX software implemented on micro-controller system,
b. Ultrasonic: transmitter, receiver (amplifier, filter), logarithmic amplifier, A/D converter, ECHOFOX software implemented on micro-controller system,
c. TDR: transmitting pulse generator, sensing pulse generator, frequency control for clock oscillators, sampling receiver, amplifier, filter, A/D converter, ECHOFOX software implemented on micro-controller system,
d. Vibration: oscillator, amplifier, filter, frequency or amplitude discriminator, A/D converter,
e. Process pressure, differential pressure: capacitance/voltage converter, A/D converter,
f. capacitive sensor: capacitance/voltage converter, A/D converter.
Sensor Unit:
A unit or assembly comprising a sensor element and a sensor electronics unit.
Evaluation Unit:
An electronics assembly, such as a circuit with electronic components and associated software for processing the measured value characterizing the process quantity to obtain an output value representing the same process quantity or a physical process quantity derived from it. An evaluation unit can carry out, for example, empty/full adjustments, linearization, scaling, sensor diagnosis and generating error messages. All above-mentioned activities of an evaluation unit can be implemented as software elements on a micro-controller system.
Output Unit:
An electronics assembly such as a circuit with electronic components and associated software for outputting at least the output value as well as other existing information (e.g. on the functional status of the sensor itself). The output unit can be expanded to provide an input and output unit, additionally enabling the input of, for example, adjustment values and parameters for functional optimization of the sensor. In this case, this could also be called a communication unit, comprising, for example, a digital communication, preferably within a bus system, between the sensor and the periphery. An output unit can include a current output of between 4 and 20 mA, a relay, an IIC interface, various modems (HART, PA, FF etc.). Corresponding communication protocols (HART, PA, FF etc.) can be implemented on a micro-processor system.
Voltage Supply Unit:
An electronic assembly such as a circuit with electronic components and associated software for converting the electrical energy supplied from the outside into energy of suitable electrical voltage sources for supplying the remaining electronic units of the sensor.
Display Unit:
An electronic assembly of the sensor for displaying the output value and perhaps further data such as indications on the operational status of the sensor etc.
Operation Unit:
An electronic assembly of the sensor for inputting at least one value such as an adjustment value.
Display and Operation Unit:
A generic term for a unit comprising at least one display unit or operation unit. In one exemplary embodiment, this unit comprises both a display unit and an operation unit in a common assembly.
Physical Process Quantity:
A physical characteristic of the process which is to be determined and output by the sensor (example: pressure, filling level, temperature, flow).
Physical Measuring Quantity:
A physical quantity of the process that is directly measured. It can either be identical to the physical process quantity or may allow the physical process quantity to be derived from it. For example, in hydrostatic filling level measurement the pressure is the physical measuring quantity.
Electrical Measuring Quantity:
An electrically measurable quantity of the sensor element e.g. the capacitance value provided by a capacitive pressure measuring cell when a pressure is applied to it.
Measuring Principle:
Characterized by the physical measuring quantity. In summary, the following measuring principles may be distinguished for the purposes of the present invention: a) delay measurement of freely radiated or guided waves reflected on a filling matter, such as electromagnetic waves or ultrasonic waves, b) measurement of a capacitance of a filling matter, c) measurement of a pressure or differential pressure, d) limit level measurement by means of vibration or in a conductive way, e) temperature measurement.
Measuring Technique (Sensorics)
Characterized by the electrical measuring quantity or the manner in which the electrical measuring quantity is obtained and evaluated. A further feature of the measuring technique is the functional operation of the sensor element.
In the following, exemplary embodiments of sensors and their different characteristic quantities explained above have been listed in the table below.
physical processphysical measuringelectrical measuringmeasuringmeasuringSensorquantityquantityquantityprincipletechniqueradar fillingfilling levelreflector-delay ofdistancemeasurementlevel sensor(continuous)sensormicrowavesmeasurementof the delay ofdistanceradiated microwavesultrasonicfilling levelreflector-delay ofdistancemeasurement of thefilling level(continuous)sensorultrasonicmeasurementdelay of radiatedsensordistancewavesultrasonic waveshydrostaticfilling levelpressure of acapacitance of apressurecapacitancepressure(continuous)liquid columncapacitive pressuremeasurementmeasurement insensormeasuring cella measuring cellcapacitivefilling levelelectricalelectricalcapacitancecapacitancesensor(continuous orcapacitancecapacitancemeasurementmeasurementlimit level)between probebetween probeusing a probeand vesseland vesselvibrationfilling leveleffect of thedampening orvibrationmeasurement ofsensor(limit level)filling matterfrequency offsetmeasurementdampening oron mechanicalof a mechanicalof a mechanicalfrequency offsetvibratorsvibratorvibratorof a vibration